Sheet-fed offset printing machines comprise a plate cylinder onto which is fed a printing plate bearing a transfer image. The printing plate is clamped to a clamping rail disposed on the plate cylinder and is then drawn onto the plate cylinder by forward rotation of the plate cylinder. The process of feeding a new printing plate to the printing machine can be time consuming. In order to minimize the time required to feed the printing plate onto the plate cylinder, the prior art has provided automatic clamping apparatuses. In such apparatuses, a remotely activatable actuating means fastens and pretensions the printing plate automatically. Fully automatic printing plate changing systems also have become known. In such fully automatic systems, the entire printing plate changing operation occurs automatically. Such systems, however, are expensive and susceptible to disruption.
The prior art further has provided semi-automatic printing plate changing systems, in which the clamping and tensioning of the printing plate take place by remotely controllable actuating means, but wherein the removal of an old, used printing plate from the plate cylinder and the feeding of a new printing plate onto the plate cylinder are carried out manually. For example, DE 4,214,049 C2 discloses a semi-automatic printing plate changing system in which a pivotal, vertically displacable guard plate is positioned in front of the plate cylinder. The guard plate includes retaining devices for new printing plates to be fed to the plate cylinder. The retaining devices include a roller journalled to the lower region of the guard plate, and further include one or more suction devices in the upper region of the guard plate. After the guard plate has been raised to allow access to the plate cylinder, the guard plate is pivoted. The printing plate is introduced manually into the clamping device of the plate cylinder and then is clamped. The printing plate then is drawn onto the plate cylinder by forward rotation of the plate cylinder.
This semi-automatic printing plate feeding system suffers from a number of drawbacks. First, the printing plates are required to be bent at one end, and must include venting bores, so that the suction devices release the printing plate shortly before the bent section is reached. In addition, after the end of the printing plate has been released by the suction devices, the printing plate then is guided only by the roller. As such, the end of the printing plate sags and, in some instances, is damaged on the printing side as a result. Moreover, the suction devices are prone to soiling, thus considerably reducing the retaining force thereof.
Other apparatuses for feeding a plate cylinder to a printing plate are known. For example, the R 700 sheet-fed offset printing machine of MAN Roland Druckmaschinen AG incorporates a vertically displacable guard plate for the plate cylinder, the guard plate including, on its lower edge, two stops onto which a new printing plate can be held in a ready position. To change the printing plate, the guard plate is raised to allow access to the plate cylinder. The used printing plate is removed manually from the plate cylinder, and the new printing plate is removed from the stops and introduced into the clamping rail of the plate cylinder. During such operation, care must be taken that the plate is not damaged when it is removed from the stops and fed to the plate cylinder.
It is a general object of the present invention to provide an apparatus of simple construction for feeding a printing plate to a printing machine. It is a further general object of the present invention to provide an apparatus for feeding a printing plate to a printing machine, wherein the printing plate is minimally susceptible to damage during the feeding of the printing plate to the printing machine.